Just so you know where I’m coming from, I play a ’72 US Telecaster through a Vox AC15.
The sweet spot is the culmination of setting up your amp to best enhance and bring out the natural tone of your guitar and/or signal chain. So if you have a Fender Telecaster, for example, the sweet spot will be very different than if you’re using a Gibson Les Paul. The Tele will require a sweet spot that enhances the guitar’s toppy, twangy sweetness, while the Les Paul will produce a sweet spot that brings out that guitar’s softer, more powerful sound.
No doubt you’ve been experimenting with the knobs and switches on your amp to get different sounds when all of a sudden it sounds great. Move a dial slightly and that sound isn’t there anymore; sure it still sounds good, but it’s not hitting the sweet spot. Or if you find a sweet spot and then plug in a different guitar, you’ll find that the amp responds differently to the input signal it receives from the new guitar.
So with my Vox AC15, when I need it, I can get a variety of different sounds, all with the clarity and presence I would demand from a true Class A rig. The other great thing about this amp is character. Many people say that the Vox AC15 and, to a greater extent, the Vox AC30 tend to smooth out the sound, softening the edges and enhancing brightness. But when it’s set up for it, your Vox AC15 can be a real bitch. Try playing a high output guitar (something with P90s or Humbuckers), through a BOSS OC2 Octave pedal and driving a Vox AC15 pretty hard, there’s no sound like that, it’s brutal. It doesn’t have as high a gain as an Ibanez through a Marshall, but the clarity and depth of tone is unmatched.
Reliability has always been an issue with tube amps. For those new to the world of tube amps, here’s a quick (very quick) breakdown. Tube amps are almost always superior to tubeless or solid state amps. Simply because the tube is what produces the tone. Without tubes, your amp is simply emulating a sound, rather than creating it. This is why tube amps have sweet spots.
Like probably now, the longer you leave your amp on, the hotter the tubes will get. And the hotter the tubes get, the warmer and more pleasant the sound it creates. But valves, a bit like light bulbs, burn out. In the old days, a broken tube could wreak havoc on your amp, short your fuses, and even catch fire. Today changing a valve is as easy as changing a fuse. Once you’ve done it, it’s simple.
So the key to understanding and finding the sweet spot for your amp is experimentation.